Active matrix liquid crystal display ("AMLCD") devices offer a full color, sunlight readable alternative to cathode ray tubes for many display applications, with significant reductions in weight, power and volume.
AMLCDs typically consist of a cell of twisted nematic ("TN") liquid crystals viewed between polarizers, and an integral backlight which generates the display luminance. The TN liquid crystal cell ("LCC") rotates the polarization of incident light as a function of applied voltage. If the polarizers are oriented in parallel, the non-rotated light is transmitted (clear state) while light which is rotated by 90.degree. within the LCC is absorbed by the output polarizer (dark state). This situation is reversed if the polarizers are oriented orthogonally to each other.
Individually controlled transparent, conductive "pixel electrodes", arranged in a matrix on one of the LCC substrates, form an array of individual optical shutters (pixel elements) which collectively generate the image. The light transmission of each individual pixel element is a function of the voltage which is applied across that pixel section of the LCC by the corresponding pixel electrodes. Each of these voltages is controlled by a thin film transistor ("TFT"), which is part of an array located on the same LCC substrate as the pixel electrodes. The use of TFTs within the LCC improves contrast, widens the viewing angle and minimizes cross-talk between pixel elements.
State of the art AMLCD multifunction displays have demonstrated high quality graphics presentations. However, the relatively slow TN LCC optical response times, particularly for changes in gray scale (less than full scale), cause noticeable smearing of dynamic video images.
LCC response times are temperature dependent and tend to increase as temperature decreases. Heaters can be employed to minimize these effects. Improvements to the baseline LCC response characteristics, however, are essential to make the AMLCD technology a viable display candidate for a broad range of applications.
These lengthy LCC response times reflect the viscous fluid flow characteristics of the twisted nematic (TN) molecular structure. In the bi-level mode of operation, the optical response time of TN LCCs is on the order of 20 to 30 milliseconds (msec). Response times for gray scale operation, however, can vary to as much as 100 msec or more.
In cases where the response time exceeds the period between data updates, the contrast is degraded unless the desired transmissiveness of each pixel element remains unchanged through enough data updates for the desired gray level to be obtained. This is not possible in many dynamic situations. An order of magnitude LCC response time improvement would alleviate this problem and vastly improve the applicability of AMLCDs to demanding video requirements.
The response time of a TN device also varies inversely as the square of the cell thickness. Therefore, a reduction in cell thickness from the value of 5-7 microns typically used in a commercial products could have a considerable effect on performance. Achieving the needed order of magnitude response time improvement, however, necessitates reducing the cell thickness by almost 70%. It is not yet possible to manufacture high quality liquid crystal cells that thin.
Even ignoring the manufacturing issues, the attainable LCD contrast is a function of cell thickness, see Gooch & Tarry: "Optical Characteristics of Twisted Nematic Liquid-Crystal Films", Electronics Letters, 10 Jan. 1974. With the currently available liquid crystal materials it would be impossible to achieve the requisite amount of contrast with such a thin device.
What is therefore needed is a new LC structure, different from TN, with improved fluid flow characteristics to achieve the desired reductions in display device response time while preserving high contrast and wide angle of view.
Liquid crystal devices based on an "untwisted nematic" ("UTN") liquid crystal structure have achieved turn off times of approximately 2 msec, and turn on times of less than 200 .mu.sec. Like TN devices, these UTN cells control the polarization of incident light and are typically viewed between front and rear polarizers, forming an array of light shutters whose transmission for each pixel element is dependent on the voltage applied across that pixel section of the LCC by the transparent, conductive pixel electrode. Also similar to TN devices, the UTN devices have what is called a "director" axis. This is the axis along which the molecules of the liquid crystal align lengthwise. An orthogonal axis may be defined as projecting orthogonally from the director axis in the plane of the polarizer.
For a TN device, the director axis rotates through 90.degree. in transitioning from one of the cell substrates to the other. Impinging light should be polarized along the director axis as it is oriented near the first LCC substrate in order for it to be received and to then have its polarization vector rotated by the twisted liquid crystal molecules. In the untwisted configuration, the director axis is fixed. For this configuration, the impinging light is polarized at an angle of 45.degree. with respect to the LCC director axis, so that it can be described as having two equal components, one orthogonal to the director axis (orthogonal component) and the other along the director axis (parallel component).
The component of the impinging light that is polarized along the director axis is phase shifted while light which is polarized orthogonally to the axis is not. The amount of phase shift is a function of the material birefringence, the cell thickness and the applied voltage.
By phase shifting the parallel component, the liquid crystal rotates the polarization vector of the light wave. If a half wave phase shift is introduced into the parallel component, for example, this component is reversed in sign as, EQU sin (180.degree.+.phi.)=-sin (.phi.)
In this case, the net polarity vector of the light is reflected about the LCC director axis. As the polarity after passing through the first polarizer is 45.degree. with respect to the liquid crystal director axis, the reflection shifts the polarity by 90.degree., (i.e., to be -45.degree. with respect to the director axis).
For phase shifts of other than a multiple of a half wave, the light exiting the LC cell has various degrees of elliptical polarization. Viewed between polarizers, this elliptical polarization would appear as intermediate levels of transmission ("gray shades").
For an untwisted LCC to provide maximum display contrast, the net phase shift of the device in the unpowered state must be at least a half-wave. Applying an increasing drive voltage to the LCC would reduce the net phase shift from its maximum value (unpowered state) toward zero phase shift (which would occur at a drive level corresponding to the device "saturation voltage"). Varying the phase shift from zero to a half wave maximizes the display contrast as the device transmission (viewed between polarizers) is varied from a maximum to minimum (or vice versa).
The output polarizer (analyzer) may be set parallel to the input polarizer or orthogonal to it. The orthogonal arrangement yields a result opposite from that produced by the parallel arrangement for the same LCC state. The LCC state which would produce maximum transmittance for an assembly with the polarizers parallel to each other, produces a zero transmittance state if the polarizers are set orthogonally. If the front and rear polarizers are parallel to each other, the net device transmission is given by the formula: EQU T=T.sub.o cos.sup.2 (.phi./2)
Note that the variability of transmission as a function of phase shift permits gray scale displays with this arrangement.
While the "untwisted" device architecture offers order-of-magnitude improvements in response time compared with TN devices, the voltage required for near-zero phase shift is much higher (up to 20 volts root-mean-square (rms) compared to about three volts rms for TN devices). This voltage is beyond the capability of standard amorphous silicon thin film transistors (TFTs).
Drive problems can be alleviated by designing the untwisted cell such that the phase shift in the unpowered state is greater than one half wave. This can be done by using liquid crystal materials with higher birefringence, or by increasing the LCC thickness from that which would produce a half wave phase shift in the unpowered state. Since the saturation voltage (corresponding to near-zero phase shift) remains essentially unchanged under these conditions, changing the cell design to increase the unpowered phase shift means that the drive voltage required to change the phase shift by a half wave with respect to the phase shift corresponding to the unpowered state is reduced. With proper display system design (described below), a half-wave change in LCC phase shift with respect to the unpowered value can provide display contrast equivalent to that achieved from varying between absolute phase shifts of zero and a half wave.
A different problem for twisted as well as untwisted devices is created by light falling on the front face of the display and which enters the assembly though the polarizer. Some of this light is reflected by the LCC. With the assembly heretofore described, such light would result in the nontransmissive state not being as dark as would otherwise be possible. This lowers display contrast, particularly in high ambient light conditions.
The prior art has not solved the problems addressed herein. The patent granted to Kashnow, U.S. Pat. No. 3,912,369 entitled "Single Polarizer Reflective Liquid Crystal Display" describes a device dependent on light entering from the face of the device and reflecting from a mirror in the rear of the device. The display uses liquid crystals with a twisted molecular structure and, accordingly, does not confront or resolve the problems posed by the slow switching rates of TN liquid crystals.
In the patent granted to Adams et al., U.S. Pat. No. 3,915,553, a color filter system is taught which is not optimized to change states rapidly in response to an electrical input. U.S. Pat. No. 4,093,356, granted to Bigelow, teaches a liquid crystal display which uses twisted nematic liquid crystal devices, change of state time of which is limited by the relatively slow times exhibited by twisted nematic liquid crystal devices generally.
The patent granted to Shanker et al., U.S. Pat. No. 4,991,924, discloses an optical switch that uses a type of liquid crystal cell which resolves incoming light into two orthogonal circularly polarized components. Because the device size is generally quite small, the teaching has been limited to fiber optic technology.
U.S. Pat. No. 4,919,522, granted to Nelson, discloses an optical switch that works with a type of liquid crystal cell that has birefringence along two different sets of axes, that is it phase shifts one light wave with respect to the other. The light is directed along a chosen path by splitting the light into two paths, one of which is blocked by a polarizer. Nelson does not address the issue of achieving variable levels of transmission, nor does he consider the problem of achieving a clean switching result with a relatively low voltage source.
The patent granted to Kalmanash, et al., U.S. Pat. No. 4,770,500 (assigned to the assignee of the present invention) discloses a device designed to switch states so that one color or another is emitted. It does not address the display of gray scales by the LCD. Although there is a signal level which causes the colors to be mixed, resulting in a third color, there is no independent intensity control associated with the LCD.
U.S. Pat. No. 5,005,952, granted to Clark et al., teaches a polarization controller. By arranging a number of liquid crystal variable polarity shifters, the user is able to shift the polarity of light as it passes through the device. This device, however, is not designed to control the amount of light passing through but rather, is designed to control the polarization of the light it emits.